The present invention relates to low cost catalytic articles and methods for treating a fluid stream, e.g., a gaseous fluid stream. Among other things, the articles and methods disclosed herein are well suited for converting pollutant components in exhaust streams produced by small engines to innocuous components. The exhaust gases of internal combustion engines, including small engines, are known to contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx) that foul the air.
More stringent emission regulations for devices powered by small internal combustion engines are increasingly being mandated by various regulatory agencies. By small engines, it is meant that the engines, usually two-stroke and four-stroke spark ignition engines, have a displacement of less than about 75 and preferably less than 35 cubic centimeters. Such engines (“utility engines”) are found, in particular, in gasoline-engine powered lawn mowers, motorized chain saws, portable generator units, snow blowers, grass/leaf blowers, string mowers, lawn edgers, garden tractors, motor scooters, motorcycles, mopeds, and like devices. Such engines provide a severe environment for a catalytic exhaust treatment apparatus. This is because in small engines, the exhaust gas contains a high concentration of unburned fuel and unconsumed oxygen. Since the users of many of such devices (e.g., motorized saws, lawn mowers, string cutters) work in close proximity to the devices, the concern for reducing the emissions is heightened.
Exhaust treating catalyst articles offer one solution toward reducing emissions from devices powered by small engines. However, practical integration of catalytic articles into such devices can be difficult because the operating conditions for small engines pose difficult design challenges.
First, the catalyst article must be durable. In comparison to devices powered by larger engines (e.g., an automobile), devices powered by smaller engines are less able to absorb and diffuse the vibrations caused by the engine. The vibrational force in a two-stroke engine can be three or four times that of a four-stroke engine. For example, vibrational accelerations of 70 G to 90 G (G=gravitational acceleration) at 150 hertz (Hz) have been reported for small engines. The harsh vibration and exhaust gas temperature conditions associated with small engines lead to several modes of failure in the exhaust gas catalytic treatment apparatus, including failure of the mounting structure by which a catalyst member is secured in the apparatus and consequential damage or destruction of the catalyst member due to the mechanical vibration and to flow fluctuation of the exhaust gas under high temperature conditions. In addition, small engines provide less design flexibility with regard to the placement of the catalytic article. In devices powered by small engines, the close proximity of the catalytic article to the engine exposes the article to intense vibrations. Furthermore, small engines are characterized by high temperature variations as the load on the engine increases and decreases. Accordingly, a catalyst member used to treat the exhaust of a small engine is typically subjected to greater thermal variation and more vibration than the catalytic converter on an automobile, and these conditions have lead to spalling of catalytic material.
Second, the catalytic articles preferably accommodate high flow rates since the majority of small engine platforms exhibit high space velocities due to the limited size of the mufflers employed on these engines. For instance, a small engine having a displacement of 50 cubic centimeters operating with a maximum of 8,000 rpm typically has an exhaust output of 12,000-15,000 L/h. Catalyst articles that significantly restrict the flow rate of the exhaust stream are less desirable since higher backpressures within the exhaust system reduce the engine's operating efficiency. Moreover, as a result of the high flow rate of exhaust stream through the catalyst article, the catalyst composition employed must be highly active and optimally disposed within the article to ensure adequate pollutant conversions.
Third, the catalyst articles are preferably lightweight and occupy small volumes since many of the devices powered by small engines are handheld tools, e.g., weed trimmers, chainsaws. Excessive weight or unwieldy protrusions from such devices negatively restrict the applications that the devices were designed for.
Fourth, the cost of the emissions treatment system cannot significantly increase the overall cost of the device to ensure that the device remains competitive on the marketplace. Small engines typically power moderately priced devices. Accordingly, a need has arisen to design a catalytic article for treating the emissions of devices powered by small engines which meets expected standards, yet minimizes the added cost to the device.
Catalysts useful in small engine applications are described in U.S. Ser. No. 08/682,247, hereby incorporated by reference. Briefly such catalysts comprise one or more platinum group metal compounds or complexes, which can be on a suitable support material. Suitable support materials include refractory oxides such as alumina, silica, titania, silica-alumina, aluminosilicates, aluminum-zirconium oxide, aluminum-chromium oxide, etc. The catalytic materials are typically used in particulate form with particles in the micron-sized range, e.g., 10 to 20 microns in diameter, so that they can be formed into a slurry and applied as a washcoat on a carrier member. Suitable carrier members may be employed, such as a honeycomb-type carrier of the type having a plurality of fine, parallel gas-flow passages extending therethrough from an inlet or an outlet face of the carrier so that the passages are open to fluid-flow therethrough. The coater carrier is disposed in a canister suited to protect the catalyst member and to facilitate establishment of a gas flow path through the catalyst member, as is known in the art.
Commonly assigned U.S. Publication No. 2004/0087439, published May 6, 2004, discloses a catalyzed metallic substrate useful as part of exhaust systems which can be used with small engines for applications such as motorcycles, lawn mowers, chain saws, weed trimmers, and the like.
Commonly assigned U.S. Publication No. 2004/0038819, published Feb. 26, 2004, discloses a pliable refractory metal carrier may have coated thereon an anchor layer to improve adherence to the carrier of a catalytic coating. The conformable catalyst member may be bent to conform to a curved or bent exhaust pipe within which it is mounted.
Commonly assigned U.S. Publication No. 2002/0128151, published Sep. 12, 2002, discloses electric arc spraying a metal onto a foam substrate to produce an anchor layer on the substrate that serves as a surprisingly superior intermediate layer for a catalytic material deposited thereon. Spalling of catalytic material is resisted even when subjected to the harsh conditions imposed by small engines or in a close-coupled position for a larger engine. It is further disclosed that the catalytic coating can be applied to substrates such as foam, corrugated foils, or screens.
Typically, two separate units, a muffler and a catalytic converter, are employed in engines to minimize emission noise and air pollution, respectively, at two separate stages of the exhaust system. Mufflers typically are designed in one of three ways: (a) with staggered baffles; (b) with sound defeating angling; or (c) with fiberglass packing. Staggered baffled mufflers are the most commonly used in the industry because they are efficient, inexpensive and easy to manufacture.
Catalytic converters, on the other hand, are typically designed in two ways: (a) with a honeycomb material; or (b) with beads. Both the honeycomb material and the beads are used as supports which are coated with a catalytic substance which causes the undesirable and harmful compounds in the exhaust gas emission stream to be converted in a predetermined catalytic reaction into harmless components.
Hence, treatment of noise and air pollution within an exhaust gas emissions stream is accomplished conventionally by these two separate devices, each acting independently of one another. Accordingly, the conventional catalytic converter does not substantially silence exhausting emissions and the conventional muffler does not catalytically treat exhausting emissions.
The need to simultaneously reduce both noise and polluting emissions is of particular importance to manufactures of small internal combustion engines, such as those for tractors, lawn and yard maintenance equipment, motor bikes, scooters, snow and leaf blowers and other power equipment where there is an increasing demand for reducing emissions and noise levels. Most of such small engines are equipped with a small muffler to control noise levels. Since existing small engine manufactures do not wish to change muffler design, a typical design for such a unit is to include a small metal substrate catalyst for controlling harmful emissions in the existing muffler. In that case, the muffler and catalyst are not independent of each other.
U.S. Pat. No. 6,040,266 discloses an acoustic baffle/catalyst foam support material and a method of producing an acoustic baffle/catalyst foam support material. According to the patent polyurethane open cell foam material is provided having a density between 10-100 ppi. The polyurethane foam is infiltrated with a resin material to produce an impregnated foam. The impregnated foam is pyrolized to form a carbon skeleton forming a carbon foam. The carbon foam skeleton is coated with one or more of the following group of materials to a relative density of five to thirty five (5-35) percent, the group of materials being SiC, Si3N4, Mo2Si2 or high temperature metal. A platinum group metal catalyst is then coated onto the metal coated foam.
It is an object of the-present invention to provide a muffler devise for the simultaneous reduction of noise levels and pollutants, such as hydrocarbons, carbon monoxide and nitrogen oxides, from an exhaust gas stream.